I’m working on a railroad engine project and I need some ideas on how to cast the wheels. I’m working in a larger scale 1:22.5 so the main wheels will be almost 2" in diameter. Unfortunately, the wheels are not solid, but have 11 spokes. I’d like to reproduce them in resin, but I don’t have a pressure pot so I am worried about voids.
I’ve heard about mash molding, but I am not sure of the process or if it would work in this application. I could make the wheels in halves then glue them together, but I’d rather make them in one solid piece.
The wheel shouldn’t be any problem casting in resin without a pressure pot. A pressure pot is used to force resin into complex one or two part molds. I have made copies of wheels without using a pressure pot.
Mash molding is used to thermaform parts not using a a vacuum form machine. I wouldn’t work for your wheels. You would need a vacuuform machine.
There is another technique called “mash molding” that is not thermaforming. I’ve heard what you describe called “plunge molding” too.
The resin varient is mostly useful for small parts. Larger resin parts generally need porting and venting with two part molds. That’s why I asked whether it was a working model or static, as that will determine where and how it is ported.
Resin fluidity is also critical.
Pressure pots and vacuum chambers are nice to have, but not necessary if you design the mold properly.
A vacuum chamber is highly useful for de-airing some mold making silicone rubbers, though.
This will be a static model. I was hoping to use Dow Corning 3110 RTV for the mold, so I wouldn’t need a vac chamber. Unfortunately it’s rather expensive unless I can get it in smaller quantities.
Are resins with longer pot times stronger than resins with shorter pot times? I was thinking to imbed metal hoop in the wheel for support if the resin would sag over time. If I can’t get metal into the wheel, I would just support the whole model from underneath.
Smooth-On makes several good RTV rubbers for modeling. They come in 1 pound kits at about 25 bucks per. You will be able to make several RTV molds 3 to 4 inches square and 1 to 1/1.5 inches deep. Their OOMOO product flows pretty well and doesn’t require vacuum degassing.
They also make some good resins, again about 25 bucks per pound. I use their 5 minute Smooth-Cast stuff and it casts smooth with no bubbles. I have just started pressure casting and the results are even better.
They offer a couple of starter/introduction packages which have all you need to get started
Google Smooth-On. Ask your question about strength of their help desk. They are very helpful and informative. Check their distributor list for over the counter sales. They also mail order fromt he main office.
Looking at your photo, I’d say this could easily be reproduced in resin with a two-part mold. I know someone’s gonna give me grief over daring to mention this in the course of casual conversation, but I just put together a small book all about mold making and resin casting (see my website), which has several examples on this stuff, which should give you all the info you need to reproduce the wheel in resin. Additionally, if you can send me a photo of the back of the wheel, I might be able to help you lay out the plan for how your particular mold could be made.
(EDIT Jan 27) I got to thinking this morning, if that wheel is basically flat on the backside, the mold could easily be a one piece, open-face mold -even better! And far less chance of bubbles, etc. This would be a piece of cake.
Yes Ed, I’ve been on Smooth-On’s website before, but they have so many products, I wasn’t sure which would be best. Thanks for the suggestions!
I had no idea that Bare Metal sold resin and RTV, thanks Triarius!
Thanks for the idea, ModelNerd. The back side of the wheel is flat, but the spokes are beveled on front and back. An open face mold would be easy to work with, and I could always use a needle file to bevel the back side of the spokes. I’ll give it a try once I get the master finished.
